In processing equipment, such as platen presses for producing decorative melamine and formaldehyde laminates, requiring alternate heating and cooling modes, most of the energy requirements are for heating and processing equipment rather than heating the material being processed. Furthermore, the alternate heating and cooling of such a heat transfer fluid adds substantially to the energy requirements of the process.
Early efforts to conserve heat were relatively simple. For example, at the end of a heating cycle, the hot fluid was set aside for use in heat-up of the next heating cycle and in like manner, at the end of a cooling cycle, the cold fluid was set aside and stored for use in cool-down at the start of a subsequent cooling cycle. This was facilely accomplished with an unpressurized water by use of separate vessels for the hot and cold fluid, however, with a pressurized water, the same vessel must be used, alternating hot and cold fluids and thereby introducing inefficiencies as a result of cross mixing.
In U.S. Pat. No. 3,109,486, there is disclosed a system including a "regenerative" section which contained relatively cooler water during the heating cycle which, on the start of a cooling cycle, is pumped through a heat exchanger for further cooling and thence through the processing equipment wherein heat is exchanged by the fluid while cooling the equipment. The fluid is then returned to the regenerative section. When the water temperature leaving the processing equipment becomes too low to be economically stored, the regenerative section is then by-passed. Thus, at the end of the cooling cycle, the regenerative section contains hot water originally in the piping and equipment plus some additional water which has first been cooled and then reheated in cooling equipment. The cycle is similar on heating, except that relatively warmer water in the regenerative section is upwardly displaced into the main section of the accumulator with the hottest water from the accumulator being circulated through the press, cooled and returned to the regenerative section.
The heat saved during a heating cycle is readily calculated by multiplying the mass of high pressure water in the regenerative section by the specific heat and the temperature difference between the start and finish of the heating cycle. The efficiency is adversly affected by using hottest water at the beginning of a heating cycle and cooling water before introduction during a cooling cycle, i.e., inefficiencies result by using heated or cooled water over large temperatures.